A mobile communication system is required to compress a speech signal to a low bit rate for effective use of radio resources. Further, improvement of communication speech quality and realization of communication services of high fidelity are demanded by users. To meet these demands, it is preferable to use wideband speech (7 kHz signal band) of wider bands than narrowband speech (3.4 kHz signal band) used in conventional speech communication.
A technique referred to as “subband coding” is known as a method of encoding wideband signals. Subband coding refers to dividing input signals into a plurality of bands and encoding each band independently. Each band is down-sampled after the band division, and so the total number of signal samples is the same as before the band division is carried out. For the band division, a QMF (Quadrature Mirror Filter) is used in many cases. The QMF divides a signal band into two, and aliasing distortion of the low band filter and the high band filter cancel each other. For this reason, there are advantages that, for example, the cut-off characteristics of a filter need not to be so steep.
Typical coding schemes using the QMF include G.722, which is standardized by the ITU-T (International Telecommunication Union-Telecommunication Standardization Sector). G.722 is also referred to as SB-ADPCM (Sub-Band Adaptive Differential Pulse Code Modulation), and refers to dividing an input signal of 16 kHz sampling frequency into two bands, the low band signal (8 kHz sampling frequency) and the high band signal (8 kHz sampling frequency), through the QMF, and quantizing the signals of the respective bands by ADPCM. The low band signal is quantized at four to six bits per sample and the high band signal is quantized at two bits per sample, and the bit rates support three kinds of 48 kbits/sec (upon quantization of the low band signal at four bits per sample), 56 kbits/sec (upon quantization of the low band signal at five bits per sample) and 64 kbits/sec (upon quantization of the low band signal at six bits per sample).
For example, there is a technique of carrying out band division of a wideband signal to the low band signal and the high band signal through the QMF and carrying out CELP (Code Excited Linear Prediction) coding of the low band signal and the high band signal (for example, see Non-Patent Document 1). This technique realizes high speech quality coding at a bit rate of 16 kbits/sec (12 kbits/sec for the low band signal and 4 kbits/sec for the high band signal). Further, the sampling frequency for the low band signal and the high band signal is half the sampling frequency for an input signal, and, compared to cases where the input signal is encoded without carrying out band division, the amount of operation in the processing (for example, convolution processing) requiring the amount of operation proportional to the square of the signal length becomes little, so that it is possible to realize a less amount of operation.
Further, there is a technique of encoding the high band of a spectrum with high efficiency utilizing the low band of the spectrum and realizing lower bit rates (see Non-Patent Document 2).    Non-Patent Document 1: “Scalable Wideband Speech Coding using G.729 as a component,” Kataoka et al., the Institute of Electronics, Information and Communication Engineers paper D-II, March 2003, Vol. J86-D-II, No. 3, pp. 379 to 387.    Non-Patent Document 2: “A 7/10/15 kHz bandwidth scalable coder using pitch filtering spectrum coding,” Oshikiri et al., Annual Meeting of Acoustic Society of Japan Article 3-11-4, March 2004, pp. 327 to 328.